1. Field of the Invention
The present invention relates to a laser apparatus capable of outputting from a laser medium a laser beam with a high peak intensity and a small pulse width, a laser beam with a low peak intensity and a large pulse width, and a continuous laser beam respectively, as desired.
2. Description of the Related Art
FIG. 34 is a cross-sectional view illustrating a Q-switched CO.sub.2 laser apparatus having functions equivalent to those obtained by a conventional Q-switched CO.sub.2 laser apparatus described for example in FIG. 4 of the paper published in Applied Optics, Vol. 35, No. 27, Sep. 1996, p. 5383. As shown in FIG. 34, the conventional Q-switched CO.sub.2 laser apparatus includes a total reflection mirror 1 in concave form, a partial reflection mirror 2 in concave form, Brewster windows 4a and 4b, a quarter-wave plate 5, a discharge tube 6, an electrooptical modulator 7, and a pulse generator 8.
In FIG. 34, the total reflection mirror 1 is a mirror formed of Cu in concave shape for example. The partial reflecting mirror 2 is a mirror which is formed of ZnSe in concave shape, for example, and which is disposed at a location opposite to the total reflection mirror 1. The partial reflection mirror 2 and the total reflection mirror 1 form a stable type laser resonator.
In FIG. 34, a laser beam 3 is generated in the laser resonator. Only a P polarization component of the generated laser beam can pass through the Brewster windows 4a and 4b, and an S polarization component thereof is reflected by them.
The electrooptical modulator 7 is made of, for example, CdTe. The pulse generator 8 generates3 a voltage signal whose amplitude periodically changes ir a binary fashion. The generated pulse voltage is applied to the electrooptical modulator 7. The laser beam is partially output, as denoted by reference numeral 9 in FIG. 34, to the outside of the laser apparatus through the partial reflection mirror 2.
The operation of the conventional Q-switched CO.sub.2 laser apparatus in a Q-switched pulse mode is described below.
FIGS. 35 and 36 conceptually illustrate the operation of the conventional laser apparatus.
FIG. 37 illustrates a typical output characteristic of the conventional laser apparatus in the Q-switched operation mode.
First, the operation is described with reference to FIG. 35 for the case where no voltage is; applied from the pulse generator 8 to the electrooptical modulator 7.
In FIG. 35, only the P polarization component of the linearly polarized laser beam 3 can pass through the discharge tube 6 disposed between the Brewster windows 4a and 4b, so that the laser beam 3 becomes a beam circularly polarized in a counterclockwise direction after passing through the quarter-wave plate 5.
Because no voltage is applied to the electrooptical modulator 7, the laser beam 3 passes through the electrooptical modulator 7 while maintaining the circular polarization in the counterclockwise direction. After passing through the electrooptical modulator 7, the laser beam 3 is reflected by the total reflection mirror 1 and becomes a beam circularly polarized in a clockwise direction.
The laser beam 3 again passes through the electrooptical modulator 7 while maintaining the circular polarization in the clockwise direction, and further passes through the quarter-wave plate 5. As a result of passage through the quarter-wave plate 5, the laser beam becomes a linearly polarized beam comprising an S polarization component. However, the S polarization component of the laser beam cannot pass through the discharge tube disposed between the Brewster windows 4a and 4b. Therefore, no laser oscillation occurs when no voltage is applied to the electrooptical modulator 7.
Referring now to FIG. 36, the operation is described below for the case where a quarter-wave voltage is applied from the pulse generator 8 to the electrooptical modulator 7. Herein, the quarter-wave voltage refers to a voltage which causes the laser beam passing through the electrooptical modulator 7 to be modulated in phase by a quarter of the wavelength.
In this case, the laser beam 3 passes through the quarter-wave plate 5 and becomes a beam circularly polarized in a counterclockwise direction. The laser beam 3 circularly polarized in the counterclockwise direction then passes through the electrooptical modulator 7 and becomes a linearly polarized beam comprising an S polarization component. The laser beam 3 is then reflected by the total reflection mirror 1 and again passes through the electrooptical modulator 7. The laser beam 3 is changed into a beam circularly polarized in a clockwise direction during passing through the electrooptical modulator 7. The laser beam 3 further passes through the quarter-wave plate 5 and changes into a linearly polarized beam comprising a P polarization component. This laser beam 3 can pass through the discharge tube 6 disposed between the Brewster windows 4a and 4b and thus can reach the partial reflection mirror 2. Therefore, in this case, laser oscillation occurs in a Q-switched pulse mode.
As described above, a Q-switched laser beam 9 with a high peak intensity and a small pulse width can be output as shown in FIG. 37 by applying a periodically varying voltage in a binary fashion to the electrooptical modulator 7 from the pulse generator 8. This technique is generally called Q switching.
If the Q-switched laser beam 9 output from the pulse generator 8 is focused through a lens or the like, it is possible to obtain a laser beam with a high energy density, which can be used to efficiently make a hole in an object.
In the specific example shown in FIG. 37, each Q-switched pulse generated at a repetition frequency of 1 KHz has a peak power of 1.8 MW and a full width at half maximum power of 30 ns, and thus a laser energy of about 60 mJ is output per cycle.
The operation of the Q-switched CO.sub.2 laser apparatus is described below for the case where a continuous laser beam is generated, or a laser beam with a low peak power and a large pulse width is generated.
FIG. 38 illustrates the operation of the conventional Q-switched CO.sub.2 laser apparatus.
FIG. 39 illustrates a typical waveform obtained in a pulse-mode operation using the conventional laser apparatus.
As shown in FIG. 38, the laser resonator of the Q-switched CO.sub.2 laser becomes equivalent to a conventional laser resonator with a simple structure comprising only a total reflection mirror 1 and a partial mirror 2. In this case, a quarter-wave voltage is applied from the pulse generator 8 to the electrooptical modulator 7 from the beginning before laser oscillation occurs.
Therefore, a laser beam 9 is output through the partial reflection mirror 2 in either a continuous oscilation mode or a pulse oscillation mode depending on whether continuous power or power in a pulse manner is applied to the discharge tube 6. The waveform of the pulse operated by pulse mode has a lower peak power and a greater pulse width than obtained in the Q-switched mode.
If the laser beam 9 output by means of Q-switched operation using the conventional Q-switched CO.sub.2 laser apparatus is used in a laser machining process such as a hole making process, the peak power is too high to properly machine an object. The high peak power often causes damage to a part other than the object to be machined.
If the peak power is reduced to a level low enough to avoid the above problem, the laser energy of each pulse decreases by an amount corresponding to the reduction in the peak power and thus it becomes impossible to make a desired hole in the object.
One technique to solve the above mentioned problem is to combine a Q-switched laser apparatus with a non-Q-switched laser apparatus capable of generating a continuous laser beam or a laser beam in the form of a pulse. In this case, although it becomes possible to machine a variety of objects in a desired fashion, another problem occurs in the laser apparatus itself.
The average output power in the non-Q-switched continuous or pulse operation is 10 to 20 times greater than that obtained in the Q-switched operation. This means that, in the non-Q-switched operation, a laser beam with very high power always passes through the electrooptical modulator 7.
For example, in the Q-switched operation, if the repetition frequency is set to 1 kHz, then the peak power becomes 1.8 MW and the full-width at half maximum becomes 30 ns. Furthermore, if the laser beam output energy per cycle is 60 mJ, then the average output power becomes 60 W. If it is assumed that the average output power in the non-Q-switched continuous or pulse mode is 10 times that in the Q-switched mode, then the average output power in the non-Q-switched continuous or pulse mode becomes 600 W.
In most cases, the electrooptical modulator 7 used in the Q-switched CO.sub.2 laser apparatus is made of CdTe. However, the maximum laser beam power that the electrooptical modulator made of CdTe can handle is determined by the characteristics of CdTe and is as low as about 60 W. Therefore, if the laser beam with an average power of 600 w is generated in the non-Q-switched continuous or pulse mode and is passed through CdTe, the average power exceeds the maximum allowable power of CdTe. For the above reason, when a laser apparatus which operates in the Q-switched mode and a laser apparatus which operates in the non-Q-switched continuous or pulse mode aire combined together, although the system can operate normally for a some duration after the system is started, the electrooptical modulator 7 is broken or the operation becomes unstable after a while, and thus it becomes difficult to perform a laser machining process.